Multi-line addressing techniques for liquid crystal displays (LCDs) have been described, for example in US2004/150608, US2002/158832 and US2002/083655, for reducing power consumption and increasing the relatively slow response rate of LCDs. However these techniques are not suitable for OLED displays because of differences stemming from the fundamental difference between OLEDs and LCDs that the former is an emissive technology whereas the latter is a form of modulator. Furthermore, an OLED provides a substantially linear response with applied current and whereas an LCD cell has a non-linear response which varies according to the RMS (root-mean-square) value of the applied voltage.
Displays fabricated using OLEDs provide a number of advantages over LCD and other flat panel technologies. They are bright, colourful, fast-switching (compared to LCDs), provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) LEDs may be fabricated using materials including polymers, small molecules and dendrimers, in a range of colours which depend upon the materials employed. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343; and examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507.
A typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material, and the other of which is a layer of a hole transporting material such as a polythiophene derivative or a polyaniline derivative.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixellated display. A multicoloured display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image. Other passive displays include segmented displays in which a plurality of segments share a common electrode and a segment may be lit up by applying a voltage to its other electrode. A simple segmented display need not be scanned but in a display comprising a plurality of segmented regions the electrodes may be multiplexed (to reduce their number) and then scanned.
FIG. 1a shows a vertical cross section through an example of an OLED device 100. In an active matrix display part of the area of a pixel is occupied by associated drive circuitry (not shown in FIG. 1a). The structure of the device is somewhat simplified for the purposes of illustration.
The OLED 100 comprises a substrate 102, typically 0.7 mm or 1.1 mm glass but optionally clear plastic or some other substantially transparent material. An anode layer 104 is deposited on the substrate, typically comprising around 150 nm thickness of ITO (indium tin oxide), over part of which is provided a metal contact layer. Typically the contact layer comprises around 500 nm of aluminium, or a layer of aluminium sandwiched between layers of chrome, and this is sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal are available from Corning, USA. The contact metal over the ITO helps provide reduced resistance pathways where the anode connections do not need to be transparent, in particular for external contacts to the device. The contact metal is removed from the ITO where it is not wanted, in particular where it would otherwise obscure the display, by a standard process of photolithography followed by etching.
A substantially transparent hole transport layer 106 is deposited over the anode layer, followed by an electroluminescent layer 108, and a cathode 110. The electroluminescent layer 108 may comprise, for example, a PPV (poly(p-phenylenevinylene)) and the hole transport layer 106, which helps match the hole energy levels of the anode layer 104 and electroluminescent layer 108, may comprise a conductive transparent polymer, for example PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene) from Bayer AG of Germany. In a typical polymer-based device the hole transport layer 106 may comprise around 200 nm of PEDOT; a light emitting polymer layer 108 is typically around 70 nm in thickness. These organic layers may be deposited by spin coating (afterwards removing material from unwanted areas by plasma etching or laser ablation) or by inkjet printing. In this latter case banks 112 may be formed on the substrate, for example using photoresist, to define wells into which the organic layers may be deposited. Such wells define light emitting areas or pixels of the display.
Cathode layer 110 typically comprises a low work function metal such as calcium or barium (for example deposited by physical vapour deposition) covered with a thicker, capping layer of aluminium. Optionally an additional layer may be provided immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Mutual electrical isolation of cathode lines may achieved or enhanced through the use of cathode separators (not shown in FIG. 1a).
The same basic structure may also be employed for small molecule and dendrimer devices. Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated before an encapsulating can is attached to each to inhibit oxidation and moisture ingress.
To illuminate the OLED power is applied between the anode and cathode, represented in FIG. 1a by battery 118. In the example shown in FIG. 1a light is emitted through transparent anode 104 and substrate 102 and the cathode is generally reflective; such devices are referred to as “bottom emitters”. Devices which emit through the cathode (“top emitters”) may also be constructed, for example by keeping the thickness of cathode layer 110 less than around 50-100 nm so that the cathode is substantially transparent.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixellated display. A multicoloured display may be constructed using groups of red, green, and blue emitting pixels. In such displays the individual elements are generally addressed by activating row (or column) lines to select the pixels, and rows (or columns) of pixels are written to, to create a display. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned, somewhat similarly to a TV picture, to give the impression of a steady image.
Referring now to FIG. 1b, this shows a simplified cross-section through a passive matrix OLED display device 150, in which like elements to those of FIG. 1a are indicated by like reference numerals. As shown the hole transport 106 and electroluminescent 108 layers are subdivided into a plurality of pixels 152 at the intersection of mutually perpendicular anode and cathode lines defined in the anode metal 104 and cathode layer 110 respectively. In the figure conductive lines 154 defined in the cathode layer 110 run into the page and a cross-section through one of a plurality of anode lines 158 running at right angles to the cathode lines is shown. An electroluminescent pixel 152 at the intersection of a cathode and anode line may be addressed by applying a voltage between the relevant lines. The anode metal layer 104 provides external contacts to the display 150 and may be used for both anode and cathode connections to the OLEDs (by running the cathode layer pattern over anode metal lead-outs). The above mentioned OLED materials, in particular the light emitting polymer and the cathode, are susceptible to oxidation and to moisture and the device is therefore encapsulated in a metal can 111, attached by UV-curable epoxy glue 113 onto anode metal layer 104, small glass beads within the glue preventing the metal can touching and shorting out the contacts.
Referring now to FIG. 2, this shows, conceptually, a driving arrangement for a passive matrix OLED display 150 of the type shown in FIG. 1b. A plurality of constant current generators 200 are provided, each connected to a supply line 202 and to one of a plurality of column lines 204, of which for clarity only one is shown. A plurality of row lines 206 (of which only one is shown) is also provided and each of these may be selectively connected to a ground line 208 by a switched connection 210. As shown, with a positive supply voltage on line 202, column lines 204 comprise anode connections 158 and row lines 206 comprise cathode connections 154, although the connections would be reversed if the power supply line 202 was negative and with respect to ground line 208.
As illustrated pixel 212 of the display has power applied to it and is therefore illuminated. To create an image connection 210 for a row is maintained as each of the column lines is activated in turn until the complete row has been addressed, and then the next row is selected and the process repeated. Preferably, however, to allow individual pixels to remain on for longer and hence reduce overall drive level, a row is selected and all the columns written in parallel, that is a current driven onto each of the column lines simultaneously to illuminate each pixel in a row at its desired brightness. Each pixel in a column could be addressed in turn before the next column is addressed but this is not preferred because, inter alia, of the effect of column capacitance.
The skilled person will appreciate that in a passive matrix OLED display it is arbitrary which electrodes are labelled row electrodes and which column electrodes, and in this specification “row” and “column are used interchangeably.
It is usual to provide a current-controlled rather than a voltage-controlled drive to an OLED because the brightness of an OLED is determined by the current flowing through the device, this determining the number of photons it generates. In a voltage-controlled configuration the brightness can vary across the area of a display and with time, temperature, and age, making it difficult to predict how bright a pixel will appear when driven by a given voltage. In a colour display the accuracy of colour representations may also be affected.
The conventional method of varying pixel brightness is to vary pixel on-time using Pulse Width Modulation (PWM). In a conventional PWM scheme a pixel is either full on or completely off but the apparent brightness of a pixel varies because of integration within the observer's eye. An alternative method is to vary the column drive current.
FIG. 3 shows a schematic diagram 300 of a generic driver circuit for a passive matrix OLED display according to the prior art. The OLED display is indicated by dashed line 302 and comprises a plurality n of row lines 304 each with a corresponding row electrode contact 306 and a plurality m of column lines 308 with a corresponding plurality of column electrode contacts 310. An OLED is connected between each pair of row and column lines with, in the illustrated arrangement, its anode connected to the column line. A y-driver 314 drives the column lines 308 with a constant current and an x-driver 316 drives the row lines 304, selectively connecting the row lines to ground. The y-driver 314 and x-driver 316 are typically both under the control of a processor 318. A power supply 320 provides power to the circuitry and, in particular, to y-driver 314.
Some examples of OLED display drivers are described in U.S. Pat. No. 6,014,119, U.S. Pat. No. 6,201,520, U.S. Pat. No. 6,332,661, EP 1,079,361A and EP 1,091,339A and OLED display driver integrated circuits employing PWM are sold by Clare Micronix of Clare, Inc., Beverly, Mass., USA. Some examples of improved OLED display drivers are described in the Applicant's co-pending applications WO 03/079322 and WO 03/091983. In particular WO 03/079322, hereby incorporated by reference, describes a digitally controllable programmable current generator with improved compliance.
Overview
There is a continuing need for techniques which can improve the lifetime of an OLED display. There is a particular need for techniques which are applicable to passive matrix displays since these are very much cheaper to fabricate than active matrix displays. Reducing the drive level (and hence brightness) of an OLED can significantly enhance the lifetime of the device—for example halving the drive/brightness of the OLED can increase its lifetime by approximately a factor of four. The inventors have recognised that multi-line addressing techniques can be employed to reduce peak display drive levels, in particular in passive matrix OLED displays, and hence increase display lifetime.
MLA Addressing with Matrix Decomposition
According to a first aspect of the present invention there is therefore provided a method of driving an electro-optic display, the display having a plurality of pixels each addressable by a row electrode and a column electrode, the method comprising: receiving image data for display, said image data defining an image matrix; factorising said image matrix into a product of at least first and second factor matrices, said first factor matrix defining row drive signals for said display, said second factor matrix defining column drive signals for said display; and driving said display row and column electrodes using said row and column drive signals respectively defined by said first and second factor matrices.
In embodiments of this method factorising the image matrix into at least two factor matrices defining row and column drive signals for the display (which in embodiments may be scaled as described later) enables the drive to pixels of the display to be spread over a longer time interval, thus reducing the maximum pixel drive for a given apparent brightness, taking into account integration within a viewer's eye. Thus preferably the driving comprises driving a plurality of the row electrodes in combination with a plurality of the column electrodes. In this way advantage may be taken of correlations between the luminescence of pixels in different rows to build the required luminescent profile of each line or row of the display over a plurality of lines scan periods, rather than as an impulse in a single line scan period. Some benefit can be obtained even when the total number of line scan periods is the same as for a conventionally line-by-line scanned display.
In preferred embodiments neither of the first and second factor matrices is predefined or predetermined. Instead both the first and second factor matrices for each new image, that is they are re-calculated for each block of image data received defining an image for display.
Preferably therefore the method drives the display with successive sets of row and column signals to build up a displayed image, each set of signals defining a subframe of the displayed image, the subframes combining to define the complete desired image. Here a subframe may refer to a portion of the desired displayed image in either time and/or space but in preferred embodiments the subframes are displayed during successive time intervals, for example each analogous to a conventional line scan period, so that when rapidly successively displayed the desired pixel brightnesses are obtained.
As will be seen later, in embodiments of the method the image matrix factorisation can incorporate a degree of compression which allows essentially the same information (that is compressed to an acceptable degree) to be displayed in a shorter time or, equivalently, over the same period of time as a conventional frame period but with a reduced drive to each pixel, each line or row effectively being driven for a longer period than in a conventional display. In a colour display where the colour channels are processed (factorised) separately different degrees of compression may be applied to the different colour channels. In this case it is preferable to apply less compression to the green channel (of an RBG display) as the human eye is more sensitive to differences (errors or noise) in green level than to differences in red or blue levels.
In embodiments the number of subframes is no greater than the lesser of the number of rows and the number of columns of the display; preferably the number of subframes is less than the smaller of the number of rows and the number of columns. In some applications the flexibility to define arbitrarily what is a row and what is a column of the display may be limited by, for example, a desire for compatibility with existing designs, in which case the number of subframes is preferably no greater than (and preferably less than) either the number of rows or the number of columns of the display. Displays are envisaged in which each pixel (or sub-pixel of a colour display) is addressed by a corresponding row and column electrode and hence references to row and columns of the display can be understood as references to row and column electrodes of the display.
In embodiments of the method the first factor matrix has dimensions determined by the number of row electrodes and a number of subframes employed (which may be predetermined by hardware and/or software or which may be selectable dependent upon, say, display quality). Similarly, the second factor matrix has dimensions determined by the number of column electrodes and the number of subframes. As previously mentioned, preferably the first and second factor matrices are configured, for example by limiting the number of subframes or dimensions of the matrices, such that a peak pixel brightness of the display is reduced compared with row-by-row driving of the same display using the same image data (with the same overall frame period to display a substantially complete image from the received data). Reducing the peak pixel brightness, that is reducing the peak pixel drive, increases the overall display lifetime. Again, in an RBG display more subframes may be employed for one colour, in particular green, than another, to provide increased accuracy of green (as opposed to blue or red) rendering.
Broadly speaking the dynamic range of pixel drive/brightness is reduced by reducing the higher pixel drive signals and this increases display lifetime roughly proportionately. This is because the lifetime reduces with the square of the pixel drive (brightness) but the length of time for which a pixel must be driven to provide the same apparent brightness to an observer increases only substantially linearly with decreasing pixel drive.
In some embodiments of the method the matrix factorising comprises singular value decomposition (SVD) into three factor matrices, the first and second factor matrices and a third factor matrix, the third factor matrix being substantially diagonal (with positive or zero elements defining so-called singular values). In this case the row drive signals are defined by a combination of the first and third factor matrices and the column drive signals by a combination of the second and third factor matrices. Since these combinations give rise to matrices with either positive or negative elements embodiments of this method are best suited to liquid crystal displays (LCDs) rather than to electroluminescent displays such as OLED display. However an SVD-based method may, for example, be incorporated into an iterative scheme which forces non-negative (i.e. positive or zero) valued elements.
With SVD matrix factorisation the diagonal elements of the third matrix effectively define a weight for the corresponding values in the first and second factor matrices and thus this provides a straightforward method for, in effect, compressing the image data by reducing the number of subframes displayed. Thus in embodiments of this method selective driving of the display is employed in which row and column drive signals defined by diagonal values of the third factor matrix less than a threshold value are neglected, in effect compressing the drive signals dependent upon a threshold of the diagonal values of the third factor matrix.
In a colour display in which, say, separate factorisation is applied to red, green and blue colour channels, it is preferably to give the green channel a greater weight than the others, for example by using a lower threshold value for green or by scaling the colour channel information using respective colour channel weights before the factorisation and then scaling the results back or performing an inverse scaling operation after factorisation. An alternative approach is to weight individual red, green and blue data values differently during the factorisation procedure (which is generally applied to a single image data matrix for the combined colour channels). In practice this comprises multiplying the green data values by a greater-than-unity scaling factor (and dividing by a total weight) during the factorisation. This is mathematically equivalent to scaling up before and back after factorisation, but can reduce rounding errors where, for example, a fixed number of bits integer-type (rather than floating point) representation is employed.
Similar techniques can be employed with other factorisation methods such as the non-negative matrix factorisation (NMF) mentioned below.
In other embodiments of the method the factorising comprises QR decomposition (into a triangular and an orthogonal matrix) or LU decomposition (into upper and lower triangular matrices). However in some further preferred embodiments the image matrix factorisation comprises non-negative matrix factorisation (NMF).
Broadly speaking in NMF the image matrix I (which is non-negative) is factorised into a pair of matrices W and H such that I is approximately equal to the product of W and H where W and H are chosen subject to the constraints that their elements are all equal to or greater than zero. A typical NMF algorithm iteratively updates W and H to improve the approximation by aiming to minimise a cost function such as the squared Eucliden distance between I and WH.
Non-negative matrix factorisation is particularly useful for driving an emissive display such as an electroluminescent display, in particular an OLED display, as a simple OLED cannot be driven to produce a “negative” luminescence, and it is therefore necessary, at least for driving a passive matrix OLED display, for the elements of the first and second factor matrices to be positive or zero.
The situation is different when driving LCD displays, and also when driving active matrix OLED displays in which the circuitry associated with a pixel is designed to allow both positive and negative drive inputs, for example adding or subtracting charge from a compacitor associated with a pixel in order that the light output is the sum or integral of a series of drive input signals.
In non-negative matrix factorisation (NMF) when matrix I has dimensions m×n (row×column) matrix W has dimension m×p and matrix H has dimensions p×n where p is generally chosen to be less than both n and m. Thus W and H are smaller than I, this resulting in a compression of the original image data. Broadly speaking W can be regarded as defining a basis for the linear approximation of the image data I and in many cases a good representation of I can be achieved with a relatively small number of basis vectors since images generally contain some inherent, correlated structure rather than purely random data. This image compression is useful as it enables the image to be displayed in a smaller number of row/column drive events than would otherwise be the case (for a conventional row-by-row raster scan). This in turn means that for the same frame period each pixel can be driven for longer thus reducing the pixel drive signal necessary for the same apparent pixel brightness, and hence increasing the display lifetime. In a large display such as an active matrix display with a very large number of pixels, for example 3000 by 2000 pixels, this technique also facilitates more rapid update of the displayed data. In some instances, for example where a pre-defined graphic icon or logo is being displayed, the matrix factorisation for at least this portion of the image can be pre-calculated and stored to speed up processing of images containing the logo or icon.
It is possible to order the columns in the row matrix (and the corresponding rows in the column matrix) to give the general appearance of a scanned display. This is because a pair of sets of elements comprising a row of the first factor matrix and column of the second factor matrix can be swapped with a corresponding pair without affecting the mathematical result. Sorting the matrices to give the appearance of a scanned display is useful because a computation of the image matrix factorisation can result in arbitrary ordering of drive signals to bright areas of the display, which may change from frame to frame and which can give rise to the appearance of motion artefacts or jitter. Sorting the data in the factor matrices so that bright areas of a displayed image are generally illuminated in a single direction, from top to bottom of the display, can reduce flicker.
In embodiments of the above described methods a pixel comprises red, green and blue subpixels but although the image data comprises data for each of these colour channels it is preferable that these are treated together as a single “combined” matrix. However it is then preferable that the factorising is performed subject to a constraint that the factorisation of the matrix for one channel, in particular the green, is on average more accurate than the factorisation of the matrices for the other colour channels. Thus, for example, more subframes may be used for the green channel, and/or a lower error threshold may be applied to the green channel processing, and/or a greater weight may be given to the green channel as compared with the red/blue channels and/or less relatively compression may be applied to the green channel. This is because, as mentioned previously, the human eye is more sensitive to differences (errors or noise) in green level than to differences in red or blue levels. Similar techniques may be applied in the other aspects of the invention mentioned below, and the invention also contemplates means to put the above-described green-channel processing techniques into effect in the context of the other aspects of the invention mentioned below.
According to a second aspect of the invention there is provided a method of driving an electro-optic display, the display having a plurality of pixels each addressable by a row electrode and a column electrode, the method comprising: receiving image data for display; formatting said image data into a plurality of subframes, each said subframe comprising data for driving a plurality of said row electrodes simultaneously with a plurality of said column electrodes; and driving said row and column electrodes with said subframe data.
In embodiments formatting the image data into a plurality of subframes enables the same pixels to be drive by two (or more) subframes and hence the peak drive to be reduced for the same apparent brightness, thus extending display lifetime. Preferably the formatting comprises compressing the image data into the plurality of subframes; in some embodiments some scaling of the image or subframe data may also be applied. The compressing may, as described above, employ singular value decomposition (SVD) or non-negative matrix factorisation (NMF).
Preferred embodiments of the above described methods are particularly useful for driving an organic light emitting diode display.
In a related aspect the invention provides a driver for an electro-optic display, the display having a plurality of pixels each addressable by a row electrode and a column electrode, the driver comprising; means for receiving image data for display, said image data defining an image matrix; means for factorising said image matrix into a product of at least first and second factor matrices, said first factor matrix defining row drive signals for said display, said second factor matrix defining column drive signals for said display; and means for outputting said row and column drive signals respectively defined by said first and second factor matrices.
The invention further provides a driver for an electro-optic display, the display having a plurality of pixels each addressable by a row electrode and a column electrode, the driver comprising: means for receiving image data for display; means for formatting said image data into a plurality of subframes, each said subframe comprising data for driving a plurality of said row electrodes simultaneously with a plurality of said column electrodes; and means for outputting said subframe data for driving said row and column electrodes.
The invention further provides a driver for an electro-optic display, the display having a plurality of pixels each addressable by a row electrode and a column electrode, the driver comprising; an input to receive image data for display, said image data defining an image matrix; an output to provide data for driving said row and column electrodes of said display; data memory to store said image data; program memory storing processor implementable instructions; and a processor coupled to said input, to said output, to said data memory and to said program memory to load and implement said instructions, said instructions comprising instructions for controlling the processor to: input said image data; factorise said image matrix into a product of at least first and second factor matrices said first factor matrix defining row drive signals for said display, said second factor matrix defining column drive signals for said display; and output said row and column drive signals respectively defined by said first and second factor matrices.
The invention further provides a driver for an electro-optic display, the display having a plurality of pixels each addressable by a row electrode and a column electrode, the driver comprising; an input to receive image data for display, said image data defining an image matrix; an output to provide data for driving said row and column electrodes of said display; data memory to store said image data; program memory storing processor implementable instructions; and a processor coupled to said input, to said output, to said data memory and to said program memory to load and implement said instructions, said instructions comprising instructions for controlling the processor to: input said image data; format said image data into a plurality of subframes, each said subframe comprising data for driving a plurality of said row electrodes simultaneously with a plurality of said column electrodes; and output said subframe data for driving said row and column electrodes.
The invention further provides processor control code, and a carrier medium carrying the code to implement the above described methods and display drivers. This code may comprise conventional program code, for example for a digital signal processor (DSP), or microcode, or code for setting up or controlling an ASIC or FPGA, or code for a hardware description language such as Verilog™; such code may be distributed between a plurality of coupled components. The carrier medium may comprise any conventional storage medium such as a disk or programmed memory such as firmware, or a data carrier such as an optical medium.